Seychellum

Lore & Background

Seychellum alluaudi was first described in 1893 as a species of Deckenia by Alphonse Milne-Edwards and Eugène Louis Bouvier, with the specific epithet honoring Charles A. Alluaud. Unaware of that description, Mary J. Rathbun described it as 'Deckenia cristata' in 1894 based on specimens collected by William Louis Abbott. Rathbun later synonymised the two names in 1906. In 1995, Peter K. L. Ng, Zdravko Števčić and Gerhard Pretzmann revised the family Deckeniidae and erected the new genus Seychellum for this species, placing it in the family Gecarcinucidae; subsequent phylogenetic work moved it into the subfamily Deckeniinae within Potamonautidae.

Adults have a quadrangular, somewhat flat carapace with a rough surface and scattered tubercles, divided into distinct regions by grooves. A raised ridge runs sinuously along the front of the carapace, interrupted by a central groove and lateral grooves. The antennae are minute, the chelae are unequal in size, and the walking legs are not flattened—unlike those of Deckenia. The eggs are very large, about 3.5 mm in diameter. The species is dark yellow to brown, becoming quite brown anteriorly.

The biogeographic origin of Seychellum has been debated. Proposed explanations include submerged land bridges, ancient vicariance from a Gondwanan landmass, and trans-oceanic rafting. The most recent research favors dispersal across open ocean, possibly aided by the species' greater tolerance of salt water compared to other freshwater crabs. The time of divergence from Deckenia has been estimated independently at two different dates.

Reader's Guide

Seychellum holds significance as the only truly freshwater crab in the Seychelles, occupying a unique ecological niche in the archipelago's rainforest streams. Its closest relatives are the two species of Deckenia found in East Africa, making its presence on isolated granitic islands a notable biogeographic puzzle. The genus has been central to discussions about the dispersal of freshwater organisms across oceanic barriers, with several hypotheses—including land bridges, continental drift, and rafting—proposed to explain its distribution. The most recent research favors trans-oceanic dispersal, though uncertainty remains. Seychellum is currently listed as Vulnerable on the IUCN Red List, primarily due to its restricted geographic range and potential vulnerability to habitat decline. Despite being abundant in some locations and occurring within a protected area, four of the five IUCN criteria could not be assessed due to lack of information. The genus also illustrates taxonomic refinement: originally placed in Deckenia, it was later segregated into its own monotypic genus based on morphological differences such as leg and antenna lengths, and the absence of flattened walking legs. Its large eggs, typical of freshwater crabs, and its tolerance of salt water further distinguish it from related groups.

Did You Know?

• Seychellum alluaudi is the only true freshwater crab in the Seychelles; all other true crabs there have marine larvae.
• Its eggs are about 3.5 mm in diameter, described as 'very large' by Mary J. Rathbun.
• The species was originally described twice in two years—once as Deckenia alluaudi in 1893 and again as Deckenia cristata in 1894.
• Seychellum is more tolerant of salt water than other freshwater crab families, which may have aided its dispersal across the ocean.